Electrochemical devices utilizing an organic solvent as an ion transport medium are used in various electric or electronic devices. Electrochemical devices include batteries and electrochemical capacitors. Among them, non-aqueous electrolyte secondary batteries, in particular, have high energy densities and can be made smaller and lighter, so their research and development is actively underway.
A non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator (separating film) interposed between the positive electrode and the negative electrode. A conventional non-aqueous electrolyte is composed of a non-aqueous solvent, such as ethylene carbonate or dimethyl carbonate, and a solute containing an alkali metal salt, such as LiPF6, which is dissolved in the non-aqueous solvent. As the separator, for example, a porous film made of polyethylene resin or polypropylene resin is used.
However, since such non-aqueous electrolyte secondary batteries are configured so as to provide high voltage and high energy density, decomposition of the non-aqueous electrolyte by oxidation occurs on the positive electrode side. The decomposition of the non-aqueous electrolyte by oxidation becomes more remarkable as the battery temperature becomes higher. For example, when stored at high temperatures between 60° C. and 85° C., the non-aqueous electrolyte is decomposed by reduction or oxidation, to produce large amounts of gas.
Also, these non-aqueous electrolyte secondary batteries have recently been widely used as a power source for notebook personal computers. A notebook personal computer is usually connected to an external power source, and it is often supplied with electricity from the external power source. At this time, the non-aqueous electrolyte secondary battery with which the notebook personal computer is equipped is constantly fully charged with the electricity from the external power source. Further, when the personal computer is operated, the temperature inside the battery reaches 45° C. or higher. Keeping charging the battery to a fully charged state of 4.2 V at such temperatures is a more harsh condition than storing the battery in an environment of 60° C. after it has been fully charged. Thus, in the former condition, gas tends to be produced inside the battery.
When a battery is stored at such high temperatures and in a constantly fully charged state, large amounts of gas is produced, and battery characteristics therefore degrade. Alternatively, due to the large amounts of the produced gas, the internal pressure of the battery rises so that a safety device is actuated to interrupt the current, after which charge/discharge becomes impossible. Therefore, it is strongly desired to suppress gas production during storage in a full charged state.
In order to solve the problems as described above, for example, an additive that is capable of forming a coating film on the positive electrode and/or negative electrode is incorporated into a battery. For example, a divalent phenol derivative is added to a non-aqueous electrolyte (see Japanese Patent No. 2928779).
However, although conventionally known additives have the effect of suppressing gas production, many of them lower the electronic conductivity, thereby impairing the charge/discharge characteristics of the battery. For example, the incorporation of an additive into a battery leads to degradation in the low-temperature discharge characteristics and/or charge/discharge cycle characteristic of the battery.